Fencing systems that use a virtual barrier, rather than a physical barrier, to restrict the location and movement of animals are known in the art. There are two basic types of “virtual” fencing systems.
One type of virtual fencing system employs a buried wire that defines a containment boundary. The wire radiates a signal that is sensed by a device worn by a monitored animal. As the monitored animal approaches the boundary, the signal is sensed and the device delivers a correction (e.g., typically sound or an electric shock) to the animal to dissuade it from breaching the boundary.
The other type of virtual fencing system uses a wireless positioning system, such as GPS, to establish a boundary and determine an animal's location. In this type of system, a control unit that includes a GPS positioning receiver, a means for applying a correction, and suitable control and logic circuitry/software is typically attached to an animal's collar. In conjunction with the control unit, a user establishes a containment boundary. The boundary is defined by positional coordinates, which are obtained from the GPS positioning receiver. In use (after the boundary is defined), the control unit compares the position of the receiver (i.e., the position of a monitored animal) with the containment boundary. In some such systems, as the animal approaches a warning zone near the boundary, a warning (i.e., a sound) is delivered. If the animal continues toward the boundary, a stimulus (i.e., low-level shock) is typically administered to the animal.
One benefit of some wireless fencing systems, relative to buried-wire systems, is that the wireless fencing system has the ability to dynamically change the boundary in order to regain control of an animal after a breach. Once breach occurs in a buried-wire system, the ability to control the animal is lost. A second benefit of some wireless fencing systems over a buried-wire system is that there is no disincentive in a wireless fencing system to re-cross a breached boundary. In particular, if an animal attempts to return to the original containment zone in a buried-wire system, it will be corrected (i.e., receive a stimulus) as it nears the wire. This provides a disincentive to return to the containment zone. In contrast, in a wireless system, the boundary can be reestablished behind a returning animal so that he will not be corrected or otherwise dissuaded from returning to the original confinement zone.
There are, however, some problems and drawbacks to wireless fencing systems. One problem is that there is some range of error associated with GPS positional data. For the low-cost receivers used in consumer electronic products this error, expressed as CEP or Circular Error Probability, is typically about 2 to 3 meters. Since the same receiver, or same type of receiver, will be used to program the boundary, this error will occur in both the control unit during normal containment operations and in the embedded boundary data set that defines the containment zone. Because these errors are unpredictable and non-correlated, they will sum in unpredictable magnitude and direction. That is, whatever errors occurred when the boundary is being programming will sum with the errors whenever the GPS receiver on the monitored animal is determining its current location.
Due the fact that the errors are all in random directions away from the true (geodesic) location, the relative position of the boundary and the pet may actually appear to increase or decrease in magnitude and shift in direction each time the position is calculated from new GPS data. Many solutions have been proposed to this problem, but they are generally prohibitive in cost or complexity, while yielding only marginal improvements.
A need exists, therefore, for a wireless fencing system that avoids or at least mitigates the potentially significant positional errors associated with the prior art.